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mechanical engineering

Questions and Answers of Mechanical Engineering

Consider a 20-L evacuated rigid bottle that is surrounded by the atmosphere at 100 kPa and 258C. A valve at the neck of the bottle is now opened and the atmospheric air is allowed to flow into the
A frictionless piston-cylinder device, shown in Fig. P8-115, initially contains 0.01 m3 of argon gas at 400 K and 350 kPa. Heat is now transferred to the argon from a furnace at 1200 K, and the argon
Two constant-pressure devices, each filled with 30 kg of air, have temperatures of 900 K and 300 K. A heat engine placed between the two devices extracts heat from the high-temperature device,
A constant-volume tank contains 30 kg of nitrogen at 900 K, and a constant-pressure device contains 15 kg of argon at 300 K. A heat engine placed between the tank and device extracts heat from the
A 100-L well-insulated rigid tank is initially filled with nitrogen at 1000 kPa and 208C. Now a valve is opened and one-half of nitrogen's mass is allowed to escape. Determine the change in the
Repeat Prob. 8-106 by assuming the piston is made of 5 kg of copper initially at the average temperature of the two gases on both sides. Discuss.
The electric power needs of a community are to be met by windmills with 40-m-diameter rotors. The windmills are to be located where the wind is blowing steadily at an average velocity of 6 m/s.
What would your answer to Prob. 8 - 119 be if heat were supplied to the pressure cooker from a heat source at 180°C instead of the electrical heating unit?
Steam is to be condensed in the condenser of a steam power plant at a temperature of 50°C with cooling water from a nearby lake that enters the tubes of the condenser at 12°C at a rate of 240 kg/s
The compressed-air storage tank shown in Fig. P8 - 122 has a volume of 500,000 m3, and it initially contains air at 100 kPa and 20°C. The isentropic compressor proceeds to compress air that
The air stored in the tank of Prob. 8-122 is now released through the isentropic turbine until the tank contents are at 100 kPa and 20°C. The pressure is always 100 kPa at the turbine outlet, and
A constant-volume tank has a temperature of 600 K and a constant-pressure device has a temperature of 280 K. Both the tank and device are filled with 40 kg of air. A heat engine placed between the
In a production facility, 1.5-in-thick, 1-ft Ã—3 square brass plates (Ï = 5 532.5 lbm/ft3 and cp = 0.091 Btu/lbm·°F) that are initially at a uniform temperature of
In a dairy plant, milk at 4°C is pasteurized continuously at 72°C at a rate of 12 L/s for 24 h/day and 365 days/yr. The milk is heated to the pasteurizing temperature by hot water heated in a
Combustion gases enter a gas turbine at 6278C and 1.2 MPa at a rate of 2.5 kg/s and leave at 5278C and 500 kPa. It is estimated that heat is lost from the turbine at a rate of 20 kW. Using air
Refrigerant-134a enters an adiabatic compressor at 120 kPa superheated by 2.3°C, and leaves at 0.7 MPa. If the compressor has a second-law efficiency of 85 percent, determine (a) The actual work
Water enters a pump at 100 kPa and 30°C at a rate of 1.35 kg/s, and leaves at 4 MPa. If the pump has an isentropic efficiency of 70 percent, determine (a) The actual power input, (b) The rate of
Saturated steam is generated in a boiler by converting a saturated liquid to a saturated vapor at 200 psia. This is done by transferring heat from the combustion gases, which are at 5008F, to the
Argon gas expands from 3.5 MPa and 100°C to 500 kPa in an adiabatic expansion valve. For environment conditions of 100 kPa and 25°C, determine(a) The exergy of argon at the inlet,(b) The exergy
Nitrogen gas enters a diffuser at 100 kPa and 110°C with a velocity of 205 m/s, and leaves at 110 kPa and 45 m/s. It is estimated that 2.5 kJ/kg of heat is lost from the diffuser to the surroundings
Obtain a relation for the second-law efficiency of a heat engine that receives heat QH from a source at temperature TH and rejects heat QL to a sink at TL , which is higher than T0 (the temperature
Writing the first- and second-law relations and simplifying, obtain the reversible work relation for a closed system that exchanges heat with the surrounding medium at T0 in the amount of Q0 as well
Writing the first- and second-law relations and simplifying, obtain the reversible work relation for a steady-flow system that exchanges heat with the surrounding medium at T0 a rate of Q0 as well as
Writing the first- and second-law relations and simplifying, obtain the reversible work relation for a uniformflow system that exchanges heat with the surrounding medium at T0 in the amount of Q0 as
Heat is lost through a plane wall steadily at a rate of 800 W. If the inner and outer surface temperatures of the wall are 20°C and 5°C, respectively, and the environment temperature is 0°C, the
Liquid water enters an adiabatic piping system at 158C at a rate of 3 kg/s. It is observed that the water temperature rises by 0.3°C in the pipe due to friction. If the environment temperature is
A heat engine receives heat from a source at 1500 K at a rate of 600 kJ/s and rejects the waste heat to a sink at 300 K. If the power output of the engine is 400 kW, the second- law efficiency of
A water reservoir contains 100 tons of water at an average elevation of 60 m. The maximum amount of electric power that can be generated from this water is (a) 8 kWh (b) 16 kWh (c) 1630 kWh (d)
One method of meeting the extra electric power demand at peak periods is to pump some water from a large body of water (such as a lake) to a water reservoir at a higher elevation at times of low
A house is maintained at 21°C in winter by electric resistance heaters. If the outdoor temperature is 9°C, the second-law efficiency of the resistance heaters is (a) 0% (b) 4.1% (c) 5.7% (d) 25%
A 12-kg solid whose specific heat is 2.8 kJ/kg·8C is at a uniform temperature of 210°C. For an environment temperature of 208C, the exergy content of this solid is (a) Less than zero (b) 0 kJ (c)
Keeping the limitations imposed by the second-law of thermodynamics in mind, choose the wrong statement below:(a) A heat engine cannot have a thermal efficiency of 100%.(b) For all reversible
A furnace can supply heat steadily at a 1300 K at a rate of 500 kJ/s. The maximum amount of power that can be produced by using the heat supplied by this furnace in an environment at 300 K is (a) 115
Air is throttled from 50°C and 800 kPa to a pressure of 200 kPa at a rate of 0.5 kg/s in an environment at 25°C. The change in kinetic energy is negligible, and no heat transfer occurs during the
aSteam enters a turbine steadily at 4 MPa and 4008C and exits at 0.2 MPa and 150°C in an environment at 25°C. The decrease in the exergy of the steam as it flows through the turbine is (a) 58 kJ/kg
How much of the 100 kJ of thermal energy at 650 K can be converted to useful work? Assume the environment to be at 25°C.
A heat engine that receives heat from a furnace at 1200°C and rejects waste heat to a river at 20°C has a thermal efficiency of 40 percent. Determine the second-law efficiency of this power plant.
Consider a thermal energy reservoir at 1500 K that can supply heat at a rate of 150,000 kJ/h. Determine the exergy of this supplied energy, assuming an environmental temperature of 25°C.
A heat engine receives heat from a source at 1100 K at a rate of 400 kJ/s, and it rejects the waste heat to a medium at 320 K. The measured power output of the heat engine is 120 kW, and the
Reconsider Prob. 8-18. Using EES (or other) software, study the effect of reducing the temperature at which the waste heat is rejected on the reversible power, the rate of irreversibility, and the
A heat engine that rejects waste heat to a sink at 510 R has a thermal efficiency of 25 percent and a second-law efficiency of 50 percent. Determine the temperature of the source that supplies heat
A house that is losing heat at a rate of 50,000 kJ/h when the outside temperature drops to 4°C is to be heated by electric resistance heaters. If the house is to be maintained at 25°C at all times,
A freezer is maintained at 20°F by removing heat from it at a rate of 75 Btu/min. The power input to the freezer is 0.70 hp, and the surrounding air is at 75°F. Determine (a) The reversible power,
Show that the power produced by a wind turbine is proportional to the cube of the wind velocity and to the square of the blade span diameter.
Can a system have a higher second-law efficiency than the first-law efficiency during a process? Give examples.
A mass of 8 kg of helium undergoes a process from an initial state of 3 m3/kg and 15°C to a final state of 0.5 m3/kg and 80°C. Assuming the surroundings to be at 25°C and 100 kPa, determine the
Air is expanded in an adiabatic closed system from 180 psia and 1408F to 20 psia with an isentropic expansion efficiency of 95 percent. What is the second-law efficiency of this expansion? Take T0 5
Which is a more valuable resource for work production in a closed system - l5 ft3 of air at 100 psia and 250°F or 20 ft3 of helium at 60 psia and 200°F? Take T0 = 77°F and P0 = 14.7 psia.
A piston-cylinder device contains 8 kg of refrigerant- 134a at 0.7 MPa and 60°C. The refrigerant is now cooled at constant pressure until it exists as a liquid at 20°C. If the surroundings are at
The radiator of a steam heating system has a volume of 20 L and is filled with superheated water vapor at 200 kPa and 200°C. At this moment both the inlet and the exit valves to the radiator are
Reconsider Prob. 8 - 30. Using EES (or other) soft ware, investigate the effect of the final steam temperature in the radiator on the amount of actual heat transfer and the maximum amount of heat
A well-insulated rigid tank contains 6 lbm of saturated liquid-vapor mixture of water at 35 psia. Initially, three-quarters of the mass is in the liquid phase. An electric resistance heater placed in
An insulated piston - cylinder device contains 0.8 L of saturated liquid water at a constant pressure of 120 kPa. An electric resistance heater inside the cylinder is turned on, and electrical work
Reconsider Prob. 8-33. Using EES (or other) software, investigate the effect of the amount of electrical work supplied to the device on the minimum work and the exergy destroyed as the electrical
An insulated piston-cylinder device contains 0.03 m3 of saturated refrigerant-134a vapor at 0.6 MPa pressure. The refrigerant is now allowed to expand in a reversible manner until the pressure drops
Oxygen gas is compressed in a piston - cylinder device from an initial state of 12 ft3/lbm and 75°F to a final state of 1.5 ft3/lbm and 525°F. Determine the reversible work input and the increase
A piston - cylinder device initially contains 2 L of air at 100 kPa and 25°C. Air is now compressed to a final state of 600 kPa and 150°C. The useful work input is 1.2 kJ. AssumingThe surroundings
A 0.8-m3 insulated rigid tank contains 1.54 kg of carbon dioxide at 100 kPa. Now paddle wheel work is done on the system until the pressure in the tank rises to 135 kPa. Determine (a) The actual
An insulated piston - cylinder device initially contains 20 L of air at 140 kPa and 27°C. Air is now heated for 10 min by a 100-W resistance heater placed inside the cylinder. The pressure of air is
An insulated rigid tank is divided into two equal parts by a partition. Initially, one part contains 3 kg of argon gas at 300 kPa and 70°C, and the other side is evacuated. The partition is now
A 70-lbm copper block initially at 220°F is dropped into an insulated tank that contains 1.2 ft3 of water at 65°F. Determine (a) The final equilibrium temperature and (b) The work potential wasted
An iron block of unknown mass at 85°C is dropped into an insulated tank that contains 100 L of water at 20°C. At the same time, a paddle wheel driven by a 200-W motor is activated to stir the
A 12-ft3 rigid tank contains refrigerant-134a at 30 psia and 55 percent quality. Heat is transferred now to the refrigerant from a source at 120°F until the pressure rises to 50 psia. Assuming the
Stainless steel ball bearings (ρ = 8085 kg/m3 and cp = 0.480 kJ/kg ∙ °C) having a diameter of 1.2 cm are to be quenched in water at a rate of 1400 per minute. The balls leave the oven at a
An ordinary egg can be approximated as a 5.5-cmdiameter sphere. The egg is initially at a uniform temperature of 8°C and is dropped into boiling water at 97°C. Taking the properties of egg to
Chickens with an average mass of 1.6 kg and average specific heat of 3.54 kJ/kg·°C are to be cooled by chilled water that enters a continuous-flow-type immersion chiller at 0.5°C and leaves at
A piston - cylinder device initially contains 1.4 kg of refrigerant-134a at 100 kPa and 20°C. Heat is now transferred to the refrigerant from a source at 150°C, and the piston which is resting on a
Refrigerant-134a at 1 MPa and 100°C is throttled to a pressure of 0.8 MPa. Determine the reversible work and exergy destroyed during this throttling process. Assume the surroundings to be at 30°C.
Reconsider Prob. 8 - 48. Using EES (or other) software, investigate the effect of exit pressure on the reversible work and exergy destruction. Vary the throttle exit pressure from 1 to 0.1 MPa and
Helium is expanded in a turbine from 1500 kPa and 300°C to 100 kPa and 25°C. Determine the maximum work this turbine can produce, in kJ/kg. Does the maximum work require an adiabatic turbine?
Air is compressed steadily by an 8 - kW compressor from 100 kPa and 17°C to 600 kPa and 167°C at a rate of 2.1 kg/min. Neglecting the changes in kinetic and potential energies, determine.(a)
Reconsider Prob. 8-51. Using EES (or other) software, solve the problem and in addition determine the actual heat transfer, if any, and its direction, the minimum power input (the reversible power),
Air enters a nozzle steadily at 200 kPa and 65°C with a velocity of 35 m/s and exits at 95 kPa and 240 m/s. The heat loss from the nozzle to the surrounding medium at 17°C is estimated to be 3
Reconsider Prob. 8 - 53. Using EES (or other) software, study the effect of varying the nozzle exit velocity from 100 to 300 m/s on both the exit temperature and exergy destroyed, and plot the
Steam enters a diffuser at 10 kPa and 60°C with a velocity of 375 m/s and exits as saturated vapor at 50°C and 70 m/s. The exit area of the diffuser is 3 m2. Determine (a) The mass flow rate of the
Air is compressed steadily by a compressor from 14.7 psia and 60°F to 100 psia and 480°F at a rate of 22 lbm/min. Assuming the surroundings to be at 60°F, determine the minimum power input to the
Argon gas enters an adiabatic compressor at 120 kPa and 308C with a velocity of 20 m/s and exits at 1.2 MPa, 5308C, and 80 m/s. The inlet area of the compressor is 130 cm2. Assuming the surroundings
Steam enters an adiabatic turbine at 6 MPa, 600°C, and 80 m/s and leaves at 50 kPa, 100°C, and 140 m/s. If the power output of the turbine is 5 MW, determine(a) The reversible power output and(b)
Steam is throttled from 7 MPa and 500°C to a pressure of 1 MPa. Determine the decrease in exergy of steam during this process. Assume the surroundings to be at 25°C.
Carbon dioxide enters a compressor at 100 kPa and 300 K at a rate of 0.2 kg/s and exits at 600 kPa and 450 K. Determine the power input to the compressor if the process involved no irreversibilities.
Combustion gases enter a gas turbine at 900°C, 800 kPa, and 100 m/s and leave at 650°C, 400 kPa, and 220 m/s. Taking cp = 1.15 kJ/kg·°C and k = 1.3 for the combustion gases, determine (a) The
Refrigerant-134a enters an adiabatic compressor at 2308C as a saturated vapor at a rate of 0.45 m3/min and leaves at 900 kPa and 558C. Determine (a) the power input to the compressor, (b) the
Refrigerant-134a is condensed in a refrigeration system by rejecting heat to ambient air at 25°C. R-134a enters the condenser at 700 kPa and 50°C at a rate of 0.05 kg/s and leaves at the same
Air enters the evaporator section of a window air conditioner at 100 kPa and 27°C with a volume flow rate of 6 m3/min. Refrigerant-134a at 120 kPa with a quality of 0.3 enters the evaporator at a
Refrigerant-22 absorbs heat from a cooled space at 50°F as it flows through an evaporator of a refrigeration system. R-22 enters the evaporator at 10°F at a rate of 0.08 lbm/s with a quality of 0.3
How much exergy is lost in a rigid vessel filled with 1 kg of liquid R-134a, whose temperature remains constant at 24°C, as R-134a vapor is released from the vessel? This vessel may exchange heat
A 40-ft3 adiabatic container is initially evacuated. The supply line contains air that is maintained at 150 psia and 90°F. The valve is opened until the pressure in the container is the same as the
What is the work potential of the air in the filled container of Prob. 8-67E if it is filled in such a way that the final pressure and temperature are both the same as in the supply line? The
Steam expands in a turbine steadily at a rate of 18,000 kg/h, entering at 7 MPa and 600°C and leaving at 50 kPa as saturated vapor. Assuming the surroundings to be at 100 kPa and 25°C, determine
Air enters a compressor at ambient conditions of 15 psia and 60°F with a low velocity and exits at 150 psia, 620°F, and 350 ft/s. The compressor is cooled by the ambient air at 60°F at a rate of
Hot combustion gases enter the nozzle of a turbojet engine at 230 kPa, 627°C, and 60 m/s and exit at 70 kPa and 450°C. Assuming the nozzle to be adiabatic and the surroundings to be at 20°C,
Steam is usually accelerated in the nozzle of a turbine before it strikes the turbine blades. Steam enters an adiabatic nozzle at 7 MPa and 500°C with a velocity of 70 m/s and exits at 5 MPa and
Ambient air at 100 kPa and 300 K is compressed isentropically in a steady-flow device to 0.8 MPa. Determine (a) The work input to the compressor, (b) The exergy of the air at the compressor exit,
A 0.6-m3 rigid tank is filled with saturated liquid water at 170°C. A valve at the bottom of the tank is now opened, and one-half of the total mass is withdrawn from the tank in liquid form. Heat is
A 0.1-m3 rigid tank contains saturated refrigerant- 134a at 800 kPa. Initially, 30 percent of the volume is occupied by liquid and the rest by vapor. A valve at the bottom of the tank is opened, and
An insulated 260-ft3 rigid tank contains air at 40 psia and 180°F. A valve connected to the tank is opened, and air is allowed to escape until the pressure inside drops to 20 psia. The air
A vertical piston-cylinder device initially contains 0.12 m3 of helium at 208C. The mass of the piston is such that it maintains a constant pressure of 200 kPa inside. A valve is now opened, and
An insulated vertical piston-cylinder device initially contains 15 kg of water, 13 kg of which is in the vapor phase. The mass of the piston is such that it maintains a constant pressure of 300 kPa
Consider a family of four, with each person taking a 6-minute shower every morning. The average flow rate through the shower head is 10 L/min. City water at 15°C is heated to 55°C in an electric
Cold water (cp = 4.18 kJ/kg ˆ™ °C) leading to a shower enters a well-insulated, thin-walled, double-pipe, counter flow heat exchanger at 15°C at a rate of 0.25 kg/s and is heated to 45°C by hot

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